Error Detector (ED) instruments may measure and characterize errors in communication signals. The majority of ED equipment is designed for data that is encoded into pulses of two (binary) stable amplitudes. The majority of ED equipment is designed to bit-compare a sampled signal to only one threshold in a given moment. In contrast, pulse amplitude modulation (PAM) encodes data into pulses of M+1 stable amplitudes, wherein M+1 may be greater than two. Error detection in PAM signals thus necessitates new test equipment and new test methods.